(1) Field of the Invention
This invention relates to a process for producing a magnetic recording medium. More particularly, it relates to an improvement in a process for producing a magnetic recording medium including a step of texturing a substrate surface.
The magnetic recording medium made by the process of the present invention exhibits an enhanced durability against wear due to sliding contact of a magnetic hard disc (hereinafter abbreviated to "HD") with a magnetic head.
(2) Description of the Related Art
The recent progress of increasing the density of magnetic recording medium is remarkable. It has heretofore said that the rate of increase in the recording density of hard disc drives (hereinafter abbreviated to "HDD") is about 10 times per 10 years, but now it may be said that the rate of density increase is about 100 times per 10 years.
In HDD, there is adopted mainly a Winchester system, i.e., CSS (contact start-stop) system involving a basic operation of a sliding movement due to contact of HD with a magnetic head, floating of the head above the magnetic head and then a sliding movement due to contact of HD with the head. The CSS system has contributed in no small measure to the amazing progress in increase of recording density. However, this system has invited an attractive problem of tribology. More specifically, the amazing progress in increase of recording density has invited an increase of rotational speed of the disc and a reduction of a flying height of the magnetic head. Therefore, there is now an increasing demand for improving head-and-disc wear characteristics and stability in sliding movement, and for enhancing smoothness of the HD surface.
The key to the improvement of head-and-disc-wear characteristics lies in an increase of the tenacity of materials and a lowering of friction coefficient or an enhancement of lubrication. As regards HD, attempts have been made for lowering the friction coefficient by rendering rough the HD surface, and for coating the HD with a protective coating material such as diamond-like carbon (DLC) or a coating lubricating agent. The surface treatment for lowering the friction coefficient is referred to as "texturing treatment" and is intended to effectively reduce the contact area in the CSS system whereby the head-and-disc wear characteristics are improved. The texturing treatment comprises forming a multiplicity of bumps having peaks and indentations or valleys with predetermined height and depth on the textured HD surface, and now the texturing treatment is an essential step for the HD production.
The texturing treatment greatly depends upon the particular material of the disc substrate. For example, in the case of an aluminum disc blank having a nickel-phosphorus (NiP) coating, the surface roughening is generally effected by mechanical polishing using an abrasive grain. In the case of a glass substrate, an etching technique utilizing lithography or a combination of lithography with printing have been proposed and some of the proposed techniques have been practically adopted.
In the texturing treatment, there are problems inconsistent with each other, namely, it is difficult to precisely control the surface roughness with an enhanced production efficiency. For example, the mechanical abrasion has technical problems of over-abrading or burr occurrence, and blurring of textured areas, and the lithographic etching has a problem that the production steps are complicated.
In recent years a texturing treatment utilizing laser beam such as laser ablation and laser etching have attracted attention (see, for example, U.S. Pat. No. 5,062,021 and Japanese Unexamined Patent Publication (hereinafter abbreviated to "JP-A") 62-209,798, 3-272018 and 7-182655). The laser beam texturing is advantageous in that, first, the surface roughness of substrate can be precisely controlled, namely, the height, spacing and location of microscopic bumps created by laser beam focusing can be desirably controlled, and, secondly, the production steps are carried out in a dry state without use of any liquid and thus the HD production can be conducted without contamination of the working environment.
But, the laser beam texturing has problems such that, when the preset height or spacing of bumps is not adequate, the adsorption of the magnetic head onto the disc surface and the CSS characteristics are markedly deteriorated. Further, the flying stability of a glide head for inspecting unacceptable bumps on the disc surface is deteriorated and the head noise increases with the result of failure of inspecting the normal glide height.
Proposals of pulse oscillating laser texturing have been made. For example, U.S. Pat. No. 5,062,021 discloses the creation of crater-like bumps each comprised of a circular rim surrounding a pit and having a diameter of 2.5 to 100 .mu.m and a spot spacing of 12.7 to 25.4 .mu.m on an NiP-coated aluminum substrate by using a Q-switched pulse oscillating Nd-YAG laser with a wavelength of 1,064 nm and an oscillation frequency of 12 KHz. JP-A 7-182655 discloses the creation of bumps having a diameter of 1 to 200 .mu.m and a spot spacing of 1 to 500 .mu.m on a glass substrate by using a pulse oscillating CO.sub.2 laser with a wavelength of 10,600 nm and an oscillation frequency of 1.5 KHz.
In the proposed pulse oscillating laser texturing treatments, when the bumps spacing between the adjacent bumps on the substrate is shortened, the number of bumps to be formed in a texturing region increases and a substantial time is required for completion of texturing operation.
Working examples of pulse oscillating laser texturing on a circular strip region having a width of 17 to 20 mm spaced from the center of a disc with a diameter of 95 mm will now be specifically described.
If a pulse oscillating Nd-YAG laser with a wavelength of 1,064 nm is used at an oscillation frequency of 12 KHz as disclosed in U.S. Pat. No. 5,062,021 to form bumps with a diameter of 25 .mu.m and a spacing of 25 .mu.m between the outer peripheries of rims both in the radial and peripheral directions, about 140 thousands of bumps are formed and about 11 seconds are required for completion of the laser texturing treatment. If a pulse oscillating CO.sub.2 laser with a wavelength of 10,600 nm is used at an oscillation frequency of 15 KHz as disclosed in JP-A 7-182655 to form bumps of the same number, dimension and spacing as mentioned above, about 90 seconds are required for completion of the laser texturing treatment.
Further, if the above-mentioned pulse oscillating Nd-YAG laser with a wavelength of 1,064 nm is used at an oscillation frequency of 12 KHz as disclosed in U.S. Pat. No. 5,062,021 to form bumps with a diameter of 85 .mu.m and a spacing of 8 .mu.m between the outer peripheries of rims both in the radial and peripheral directions, about 3,500 thousands of bumps are formed and about 5 minutes are required for completion of the laser texturing treatment. If the above-mentioned pulse oscillating CO.sub.2 laser with a wavelength of 10,600 nm is used at an oscillation frequency of 15 KHz as disclosed in JP-A 7-182655 to form about 3,500 thousands of bumps of the same dimension and spacing as mentioned above, about 39 minutes are required for completion of the laser texturing treatment.
JP-A 4-281030 teaches that adsorption of a magnetic head can be reduced by the formation of bumps distributed relatively densely in the peripheral direction and relatively sparsely in the radial direction. If a pulse oscillating Nd-YAG laser with a wavelength of 1,064 nm is used at an oscillation frequency of 12 KHz as disclosed in U.S. Pat. No. 5,062,021 to form bumps with a diameter of 2 .mu.m and a spacing of 8 .mu.m and 18 .mu.m between the outer peripheries of rims in the radial and peripheral directions, respectively, as described in a working example of JP-A 4-281030, then about 1,700 thousands of bumps are formed and about 2.5 minutes are required for completion of the laser texturing treatment. If a pulse oscillating CO.sub.2 laser with a wavelength of 10,600 nm is used at an oscillation frequency of 15 KHz as disclosed in JP-A 7-182655 to form bumps of the same number, dimension and spacing as mentioned above, about 19.5 minutes are required for completion of the laser texturing treatment.
The frequency of repeated oscillation of pulse oscillation lasers is not on a high level. For example, it is at most about 20 KHz in the case of commercially available switched Nd-YAG lasers. With a frequency of repeated oscillation exeeding this level, the stability of laser output is lowered and the pulse oscillation lasers are not suitable for the laser texturing treatment.
As seen from the above-cited references, the shortening of spacings between the adjacent bumps formed on a substrate surface invites a definite reduction of workability of a laser texturing apparatus used in a step of producing a magnetic recording disc. In other words, as far as a pulse oscillating laser is used, the bump spacings cannot be reduced to a distance shorter than a certain distance without reduction of workability of the laser texturing apparatus. Thus the design of bumps to be formed on a substrate by the laser texturing is limited in view of the CSS characteristics.
As regards the height of bumps created by focusing a pulse oscillating laser beam on a substrate, said height varies depending upon the particular laser output. The height of bumps created by a laser texturing treatment is usually in the range of 10 to 50 nm, and the laser beam energy applied employed for forming bumps, for example, on a nickel-phosphorus (NiP) plated aluminum substrate is usually in the range of 1 to 10 J/spot (see IEEE, Transactions on Magnetics, vol. 31, No. 6, p2946). It is also described that a laser beam with an output of a level lower than the minimum energy for generating laser ablation is focused on a substrate such as glass or ceramic to form microscopic bumps thereon (JP-A 7-182655 and JP-A 7-156723). In these laser texturing treatments, the peak energy of pulse oscillating laser is utilized, and thus, the laser output is lowered by an attenuator.
The laser beam energy required for a laser texturing treatment is much lower than that employed for laser beam machining, and the height of microscopic bumps varies greatly depending upon the particular fluctuation of pulse oscillating laser output. When the laser beam energy is very small, microscopic bumps are occasionally not created. Therefore a laser texturing treatment by which bumps of a desired dimension can be formed by pulse oscillating laser beam even without attenuation of the peak energy or even where the laser beam is continuous wave laser.